Morphology Control of Charged Block Copolymers through Electrostatic Control of Interfaces

In the present study, we investigated the self-assembled morphologies of charged block copolymers comprising ionic liquids. To control electrostatic interactions of polymer matrix with embedded ionic liquids, two types of acid-tethered block copolymers were prepared. Increasing the amount of embedded ionic liquids in the acid-tethered block copolymers resulted in sequential morphological transitions from gyroid-to-lamellae-to-hexagonal cylinder owing to the effective swelling of ionic domains. Interestingly, when the cation-excessive nonstoichiometric ionic liquids were employed, a radical morphological change from lamellar (stoichiometric) to A15 (non-stoichiometric) structures was observed for the acid-tethered block copolymers. This was remarkable given that such phase transitions occurred despite the calculated volume fractions of ionic domains were almost identical. By varying the non-stoichiometry of ionic liquids, it has been revealed that the use of excessive cations stabilized the A15 structures by lowering the concentration fluctuation at block copolymer interfaces via electrostatic interactions. Such stabilization became pronounced when the polymer matrix binds more strongly with the cations. Computational simulations unveiled the formation of thin ionic shell layers at the block copolymer interfaces, which played a crucial role in stabilizing the low symmetry morphology. Owing to the three-dimensionally connected ionic domains in the A15 structures, enhanced ionic conductivity was achieved for corresponding block copolymer electrolytes.